Method of screening the magnetic field generated by an electrical power transmission line, and electrical power transmission line

ABSTRACT

A description is given of a method for screening the magnetic field generated by an electrical power transmission line comprising at least one electrical cable. The method comprises the insertion of the cables into a conduit comprising at least one layer of a non-grain-oriented ferromagnetic material, having a magnetic curve with a maximum value of relative magnetic permeability (μ max ) corresponding to a magnetic field value (H μmax ) of less than 1000 A/m. In one example, the ferromagnetic material is a steel with a low content of impurities.

[0001] The present invention relates to a method for screening themagnetic field generated by an electrical power transmission line. Thepresent invention also relates to a magnetically screened electricalpower transmission line.

[0002] Generally, a high-power electrical power transmission line isdesigned to withstand voltages of the order of hundreds of kV (typically400 kV) and currents of the order of thousands of amperes (typically500-2000 A). The electrical power carried in these lines can reachvalues of the order of thousands of MVA, typically 1000 MVA. Normally,the current carried is an alternating current at low frequency, in otherwords generally below 400 Hz, and typically at 50-60 Hz. In general,these lines are used for transferring power from an electrical powerstation to a city, over distances of the order of tens of km (normally10-100 km).

[0003] In a typical configuration, a three-phase line comprises threecables buried in a trench with a depth of 1-1.5 m. In the spaceimmediately surrounding the cables, the magnetic field H can reachvalues of the order of 10³ A/m. At ground level, the measurable magneticinduction can reach values of the order of 20-60 μT, depending on thearrangement of the cables with respect to each other.

[0004] Although no biological effects due to exposure to magnetic fieldsof this size generated by low-frequency (50 Hz) sources have yet beendemonstrated, there is currently a debate in the scientific communityabout a possible “safety threshold”, to be adopted by law, below whichthe probability of biological damage can be reduced to a minimum, if noteliminated. A threshold of magnetic induction on which the scientificcommunity appears to have reached agreement, and on which some nationallegislation is tending to become harmonized, is 0.2 μT. Thus the valuein question is approximately 100 times smaller than that statedpreviously. The reduction of magnetic induction levels to less than 0.2μT is certainly to be considered preferable.

[0005] The applicant has tackled the problem of screening the magneticfield generated by a high-power electrical power transmission linecomprising cables buried in a trench, with the aim of achieving magneticinduction values at ground level of approximately 0.2 μT or less.

[0006] The article by P. Argaut, J. Y. Daurelle, F. Protat, K. Savinaand C. A. Wallaert, “Shielding technique to reduce magnetic fields fromburied cables”, A 10.5, JICABLE 1999, describes some solutions forscreening magnetic fields generated by three buried cables. Inparticular, it states the results of some simulations carried out withopen-section screens (for example a sheet of ferromagnetic materialplaced above the cables) and closed-section screens (for example aconduit of rectangular section made of ferromagnetic material, placedaround the three cables). The dependency of the screening efficiency onvarious factors, such as the relative magnetic permeability of thescreening material, the thickness of material to be used and therelative positions of the cables and the magnetic screen, is analysed.According to the authors, the optimal material should have a relativemagnetic permeability in the range from 700 to 1000 and a thickness inthe range from 3 to 5 mm; in the case of a closed-section screen, theoptimal relative position is one in which the cables are approximately ⅓of the way down from the top of the screen. Also according to theauthors, attenuation factors of approximately 5-7 can be obtained withopen-section screens, factors of approximately 15-20 can be obtainedwith closed-section screens, factors of approximately 30-50 can beobtained when the closed-section screen is formed very close to thecables (for example from a sheet of ferromagnetic material wounddirectly around the three cables).

[0007] Patent application (Kokai) JP 10-117083 in the name of NipponSteel Co. provides a solution for the screening of the magnetic fieldgenerated by an electrical power transmission cable, consisting of atube of ferromagnetic material, made by winding a strip of magneticmaterial in a spiral, preferably on a tubular support, for example ametal or resin pipe. In the example described, the strip is made fromgrain-oriented steel and has a magnetic permeability which is higher ina direction parallel to the direction of winding than in the directionperpendicular thereto.

[0008] The patent EP 606884, also in the name of Nippon Steel, describesa process for producing grain-oriented silicon steel, in which the steelis subjected to a complex rolling process and subsequent stages ofannealing, with predetermined times and temperatures, in the presence ofrecrystallization inhibitors.

[0009] The applicant has observed that the solutions described in thearticle by Argaut et al. cited above do not permit the achievement ofvery high screening factors, such as those necessary to screen themagnetic field generated by an electrical transmission line.

[0010] The applicant has also observed that the solution described inthe patent application JP 10-117083 cited above provides for the use ofa grain-oriented steel. In this type of steel, the grains have adirection of orientation parallel to the direction of winding: thismakes it possible to obtain a very high magnetic flux density. It isproduced by complex production processes, which make it possible toorientate the grains only in thin sheets, having thicknesses of theorder of a tenth of a millimetre (see, for example, the patent EP 606884cited above). Because the thickness is so small, the screening tube canbe produced only by winding a steel strip in a spiral around a support,as described in patent application JP 10-117083, in order to ensuresufficient mechanical compressive strength. All this makes the processof producing a screening tube extremely complicated.

[0011] On the other hand, the applicant has found that it is possible toscreen, with an attenuation factor of the order of 100 or above, themagnetic field generated by an electrical power transmission line, byinserting the cables in a conduit comprising at least one layer ofnon-grain-oriented ferromagnetic material having a high relativemagnetic permeability in a range of magnetic field values below 1000A/m. The screening conduit can advantageously be produced by normalextrusion or rolling methods, without making use of complex productionprocesses for orientating the grains, or by winding as in the aforesaidpatent application JP 10-117083.

[0012] Here and in the remainder of the description, the term“non-grain-oriented material” denotes a material in which the crystaldomains (grains) essentially have no preferred direction of alignment.The degree of alignment can be evaluated by known methods, for exampleby optical microscopic analysis, or by X-ray diffractometric analysis.In other words, the material has not been subject to special processesof rolling and annealing, according to the methods used in theproduction of grain-oriented steel, and the only orientation which maybe present in the material is that caused by a normal extrusion orrolling process.

[0013] In a first aspect, the invention relates to a method forscreening the magnetic field generated by an electrical powertransmission line comprising at least one electrical cable, the saidmethod comprising the steps of:

[0014] inserting the said cable in a conduit comprising at least onelayer of a ferromagnetic material,

[0015] characterized in that

[0016] the said ferromagnetic material is non-grain-oriented and has amagnetic curve with a maximum value of relative magnetic permeability(μ_(max)) corresponding to a magnetic field value (H_(μmax)) lower than1000 A/m.

[0017] Preferably, the magnetic curve of the material has a maximumvalue of relative magnetic permeability (μ_(max)) corresponding to amagnetic field value (H_(μmax)) in the range from 10 A/m to 800 A/m.

[0018] Even more preferably, the magnetic curve has a maximum value ofrelative magnetic permeability (μ_(max) ) corresponding to a magneticfield value (H_(μmax)) in the range from 30 A/m to 650 A/m.

[0019] Advantageously, the maximum value of relative magneticpermeability (μ_(max)) is at least 500, being preferably in the rangefrom 700 to 5000.

[0020] Typically, the method according to the invention comprises thestep of burying the conduit in a trench of predetermined depth.

[0021] The screening layer can be produced by extrusion, or by thebending of a sheet of predetermined dimensions, for example one producedby rolling, and the subsequent welding of the sheet along itslongitudinally opposing sides.

[0022] In a preferred embodiment, the method according to the inventionadditionally comprises the step of arranging the cable or cables in theconduit in such a way that the centre of gravity of a cross section ofthe cable is in the proximity of the geometrical centre of acorresponding section of the conduit.

[0023] Advantageously, the method according to the invention canadditionally comprise the step of winding at least one elongatedelement, for example a cord, around the said at least one cable.

[0024] In a second aspect, the invention relates to an electrical powertransmission line, comprising:

[0025] a conduit comprising at least one layer of ferromagneticmaterial,

[0026] at least one electrical cable inside the said conduit,

[0027] characterized in that

[0028] the said ferromagnetic material is of the non-grain-oriented typeand has a magnetic curve having a maximum value of relative magneticpermeability (μ_(max)) corresponding to a magnetic field value(H_(μmax)) lower than 1000 A/m.

[0029] Preferably, the magnetic curve of the material has a maximumvalue of relative magnetic permeability (μ_(max)) corresponding to amagnetic field value (H_(μmax)) in the range from 10 A/m to 800 A/m.

[0030] Even more preferably, the magnetic curve has a maximum value ofrelative magnetic permeability (μ_(max)) corresponding to a magneticfield value (H_(μmax)) in the range from 30 A/m to 650 A/m.

[0031] Advantageously, the maximum value of relative magneticpermeability (μ_(max)) is at least 500, being preferably in the rangefrom 700 to 5000.

[0032] In a first example, the ferromagnetic material is a steel whosetotal content of impurities does not exceed 1.5%.

[0033] Preferably, the total content of impurities does not exceed 1%,and even more preferably it does not exceed 0.5%. The said steel with alow content of impurities can be a low-carbon steel: preferably, thecarbon content does not exceed 0.16%, and even more preferably it doesnot exceed 0.03%.

[0034] The said steel with a low content of impurities can be alow-manganese steel: preferably, the manganese content does not exceed1%, and even more preferably it does not exceed 0.5%.

[0035] Advantageously, the resistivity of the steel with a low contentof impurities is less than 20 μΩ·cm. To improve the screening effect,the grain size index G of the said steel, measured according to the ASTME-112 standard, is less than 9.

[0036] In a second example, the ferromagnetic material is a siliconsteel. Preferably, the silicon content is in the range from 1% to 4%.

[0037] Advantageously, the electrical power transmission line accordingto the invention can comprise a support for the cable or cables insidethe conduit.

[0038] In a preferred embodiment, the electrical power transmission linecomprises at least one elongated element wound in a spiral around thesaid at least one cable, for example a cord made from dielectricmaterial. Preferably, the dielectric material is selected from a groupcomprising nylon fibres, aramid fibres and polyester fibres.

[0039] Further characteristics and advantages will be more clearlyevident in the light of the detailed description of some examples of thepresent invention. This description, provided below, relates to theattached drawings provided solely by way of example and withoutrestrictive intent, in which:

[0040]FIG. 1 shows schematically in cross section an example of atransmission line according to the present invention;

[0041]FIG. 2 shows schematically a typical magnetic curve μ_(r)-H of aferromagnetic material, showing the co-ordinates of the peak of thecurve (μ_(max), H_(μmax));

[0042]FIG. 3 shows a graph of the magnetic induction B found at groundlevel when conduits made from different types of steel are used;

[0043]FIG. 4 shows a graph of the electrical losses in the conduit, fordifferent types of steel;

[0044]FIG. 5 shows the magnetic curves μ_(r)-H of different types ofsteel;

[0045]FIG. 6 shows a graph of the magnetic induction B found at groundlevel when a conduit made from non-grain-oriented silicon steel is used,as a function of the thickness of the conduit;

[0046]FIG. 7 shows a graph of the magnetic induction B found at groundlevel, as a function of the diameter of a cord wound around the cables;

[0047]FIG. 8 shows a microscopic view of the crystal domains of anon-grain-oriented steel.

[0048] With reference to FIG. 1, a line 100, suitable for electricalpower transmission at high power, comprises a conduit 101 made fromferromagnetic material, preferably having a closed cross section, and atleast one electrical cable. The section of the conduit is generallyessentially circular: sections of different shape, such as a squaresection, are not excluded. Typically, three cables 102 a, 102 b, 102 care placed inside the conduit 101, each carrying an alternating current(typically 50-60 Hz), in a three-phase line. Preferably, the cables 102a, 102 b, 102 c are arranged in a trefoil configuration, in other wordsin such a way that their geometrical centres are approximatelypositioned on the vertices of a triangle (seen in section).Advantageously, the cables are in contact with each other.

[0049] For example, the cables 102 a, 102 b, 102 c can be made fromenamelled copper Milliken conductors, with a cross section of 2500 mm²,insulated with an extruded polymer, for example cross-linkedpolyethylene (XLPE). A metal sheath can also be placed on the outersurface of the cable, to protect it from moisture. The total externaldiameter of each cable is typically in the range from 100 to 150 mm.

[0050] Preferably, the cables 102 a, 102 b, 102 c are raised above thebottom of the conduit 101 and supported by a support means 103 locatedinside the conduit 101, in such a way as to reduce the distance betweenthe centre of gravity of a cross section of the trefoil of cables andthe geometrical centre of a corresponding cross section of the conduit101. In a preferred embodiment, the support means 103 is an elongateelement wound in a spiral around the trefoil of cables.

[0051] Alternatively, the cables 102 a, 102 b, 102 c can be supported bydirect contact with the bottom of the conduit 101.

[0052] The space 104 inside the conduit 101 which is not occupied by thetrefoil of cables 102 a, 102 b, 102 c and the support 103 can be left inits air-filled state, or a, fluid, for example an inert gas, can beintroduced into it. Preferably, a small overpressure is provided insidethe conduit to impede the ingress of moisture from the exterior of theconduit 101. For example, it is possible to introduce dry nitrogen intothe inner space 104 and subject the conduit to a small internaloverpressure of approximately 0.5 bar. Thus the metal moisture-proofingsheath usually placed on the outer surface of each cable can bedispensed with.

[0053] The line 100 is typically buried in a trench, generally having adepth of not more than 0.5 m (typically 1-1.5 m). The outer wall of theconduit 101 is covered with protective material capable of preventingcorrosion, for example polyethylene or bitumen.

[0054] The thickness of the conduit 101 can be chosen from a range of 2to 14 mm, and preferably 4 to 10 mm; even more preferably, it isapproximately 8 mm. In general, the thickness of the conduit 101 ischosen in such a way as to enable the conduit to withstand stresses ofthe mechanical type (such as accidental impact of excavating equipment,for example) and also to withstand the weight of the overlying soil.

[0055] The internal diameter of the conduit 101 can be chosen from arange of 2.3 to 2.8 times the diameter of the cable carrying eachindividual phase, in such a way that the cables can be pulled throughwith sufficient ease during the laying of the line.

[0056] The applicant has found that a conduit comprising at least onelayer of non-grain-oriented ferromagnetic material, characterized by amagnetic curve having a maximum value of magnetic permeability μ_(max)corresponding to a magnetic field value H_(μmax) of less than 1000 A/m,provides a highly effective screen against the magnetic field generatedby an electrical power transmission line having a power of 1000 MVA(with a voltage of 400 kV and a current of 1500 A), as described abovewith reference to FIG. 1.

[0057] Here and in the remainder of the description, the term “magneticcurve” denotes a curve describing the variation of the relative magneticpermeability μ_(r) of a material as a function of an applied magneticfield H, as found by measurements according to the IEC 404 “Magneticmaterials” standard. In particular, according to this standard, themagnetic permeability is measured by immersing a ring of material in amagnetic field directed circumferentially with respect to it. A typicalmagnetic curve of a ferromagnetic material is shown schematically inFIG. 2, where the co-ordinates of the peak μ_(max), H_(μmax) areindicated.

[0058] Without wishing to associate himself with any particular theoryof interpretation, the applicant considers that the screening capacityof the conduit depends on the value which the magnetic field takesinside the screening material.

[0059] The value of the magnetic field inside the material will bedenoted by H_(mat) and the value of the magnetic field generated by thecables alone by H_(c). Since the material is ferromagnetic, the magneticfield H_(mat) inside it is smaller than H_(c). According to theapplicant, effective screening can be obtained by making a conduit froma material having a peak of magnetic permeability μ_(max) centred on amagnetic field value H_(μmax) close to the value which the magneticfield has inside the material (H_(mat)), in other words a value lowerthan H_(c). In the region of the peak, the magnetic permeability of thescreening material becomes very high. If the magnetic field H_(mat) hasa value close to H_(μmax) inside the material, the material has a highmagnetic permeability, and therefore a high capacity for “trapping” themagnetic field inside itself. This is equivalent to a high capacity forscreening the magnetic field produced by the cables.

[0060] The phenomenon described above is of a highly non-linear kind,owing to the dependence of the magnetic permeability μ_(r) on theapplied magnetic field: in general, it is impossible to predict thevalue which the magnetic field H_(mat) will have inside a ferromagneticmaterial immersed in an external magnetic field H_(c). According to thetype of material and the magnetic field generated by the cables, thematerial of the conduit has a certain magnetic field value H_(mat) andtherefore a certain value of magnetic permeability μ_(r): as μ_(r)increases, the conduit's capacity to trap the magnetic field alsoimproves.

[0061] The applicant has noted that the screening effect becomes moreeffective as H_(μmax) decreases. Preferably, H_(μmax) is in the rangefrom 10 A/m to 800 A/m. Even more preferably, H_(μmax) is in the rangefrom 30 A/m to 650 A/m. Advantageously, the maximum value μ_(max) ofmagnetic permeability is greater than 500, and preferably in the rangefrom 700 to 5000.

[0062] By introducing the cables 102 a, 102 b, 102 c of the transmissionline 100 into a conduit 101 comprising at least one screening layer of amaterial with the aforesaid characteristics, it is possible to obtainvalues of magnetic induction of the order of, or less than, 0.2 μT at adistance of 0.5 m from the top of the conduit.

[0063] If the maximum value of the magnetic curve of the material of thescreening layer corresponds to lower magnetic field values (aroundmagnetic field values of 200-250 A/m or below), the screening capacityincreases progressively in an exponential way, until the values ofmagnetic induction found at ground level are practically unmeasurable byordinary instruments.

[0064] The screening efficiency can decrease, however, if the screeningmaterial has a peak of magnetic permeability corresponding to magneticfield values below approximately 10 A/m. This is because, in materialshaving values of H_(μmax) as low as this, the peak of magneticpermeability can be very narrow: in other words, outside a limited rangeof magnetic field values, the magnetic permeability of the material candecrease very rapidly to values which may prove to be insufficient toscreen the intense magnetic field generated by the transmission line.

[0065] A first example of a material with the magnetic characteristicsmentioned above is a steel having a total content of impurities notexceeding approximately 1.5%, preferably not exceeding approximately 1%,and even more preferably not exceeding approximately 0.5%.

[0066] Here and in the remainder of the description, the term“impurities” denotes all elements other than iron which are present inthe steel.

[0067] More particularly, the steel of the screening layer is preferablya low-carbon steel. Here and in the remainder of the description, theterm “low-carbon” relates to a carbon content not exceeding 0.5% byweight.

[0068] Preferably, the carbon content does not exceed approximately0.16%, and even more preferably it does not exceed approximately 0.03%.

[0069] Additionally, the steel of the screening layer preferably has amanganese content not exceeding approximately 1%, and even morepreferably not exceeding approximately 0.5%.

[0070] Without wishing to associate himself with any particular theoryof interpretation, the applicant considers that the impurities present,and particularly the manganese and the carbon, reduce the magneticscreening since they are interposed, in the crystal lattice of thesteel, between the grains of iron, removing continuity from the magneticstructure of the material: thus micro-regions of low magneticpermeability are created inside the material, making the screening ofthe magnetic field less effective.

[0071] The applicant has noted that, as the grain size of the steelincreases, there is a corresponding improvement in the screeningcapacity of the layer. According to international standards, the grainsize of a steel can be determined by means of a dimensionless index G,derived from a count of the number of grains present within apredetermined area. As the grain size increases, therefore, the index Gdecreases. In some examples of steels tested by the applicant, the grainsize was evaluated by means of the dimensionless index G according tothe ASTM E-112 standard. Preferably, the size of the grains, evaluatedby means of the index G (ASTM E-112), does not exceed 9.

[0072] Preferably, the steel for an effective screening layer has anelectrical resistivity of less than 20 μΩ·cm. A low resistivityfacilitates the flow of the currents induced by the magnetic fieldgenerated by the cables on the surface of the conduit, thus improvingthe magnetic screen effect of the conduit.

[0073] A second example of a screening material with the magneticcharacteristics mentioned above is a non-grain-oriented silicon steel.To provide effective screening, the percentage of silicon in the steelcan be selected from a range from 1% to 4%.

[0074] A screening layer with the aforementioned characteristics canadvantageously be made by conventional methods. For example, the layercan be produced by extrusion, or by bending a sheet of predetermineddimensions and then welding it along its longitudinally opposing edges.This sheet can be produced, for example, by rolling.

[0075] In the particularly preferred embodiment shown in FIG. 1, thethree cables 102 a, 102 b, 102 c are arranged in trefoil configurationand raised from the bottom of the conduit 101 by at least one elongatedelement 103, for example a cord wound in a spiral around the threecables. FIG. 7 shows the result of a simulation, carried out by theapplicant, of a calculation of the magnetic induction at ground level,at a distance of 1.2 m from the top of a conduit made from low-carbonsteel, as a function of the diameter of the cord.

[0076] The following parameters were taken into account for thesimulation: external diameter of the conduit 400 mm, thickness of theconduit 10 mm, diameter of each cable 150 mm, current 1500 A, voltage400 kV.

[0077]FIG. 7 shows the decrease of the magnetic induction at groundlevel with the increase of the diameter (φ) of the cord: it can be seenthat the magnetic induction reaches a minimum value when the diameter ofthe cord is 60 mm, which corresponds to bringing the centre of gravityof the cables 102 a, 102 b, 102 c to the geometric centre of the conduit101 (seen in section).

[0078] The simulation also revealed that, when the centre of gravity ofthe cables is moved towards the geometrical centre of the conduit, thelines of flux of the magnetic induction tend to be more closely packedinside the conduit 101 and to have a more symmetrical form. Theimprovement of the symmetry of the system permits an even more effectiveselection of a steel having a peak of relative magnetic permeabilityaccording to what has been mentioned previously, since the magneticfield generated by the cables is essentially uniform over the wholeinternal surface of the conduit, as is the magnetic behaviour of thematerial from which the conduit is made. In other words, when the centreof gravity of the cables 102 a, 102 b, 102 c is brought towards thegeometric centre of the conduit 101 the material of the conduit comes tohave essentially the same relative magnetic permeability along the wholeof the conduit.

[0079] The support 103 makes it possible to reduce the losses due toparasitic currents, which are localized in the regions of the conduit101 close to the points of contact of the cables 102 a, 102 b, 102 c,owing to the spacing of the two cables 102 b, 102 c from the bottom ofthe conduit: in the upper region of the conduit 101 there is a smallincrease in the losses, due to the corresponding approach of the cable102 a. Anyhow, the overall effect is a reduction of the losses.

[0080] Advantageously, the use of an element wound around the cables 102a, 102 b, 102 c enables the cables to be kept in close contact with eachother at all times, even when they tend to become separated as a resultof thermomechanical or electromechanical stresses. By keeping the cablesin contact with each other, it is possible to reduce to a minimum thedistance between the centres of the cables, in other words between thecentres of the currents flowing in the cables, along the conduit 101,with a consequent decrease in the magnetic induction to be screened.

[0081] The diameter of the supporting cord 103 can be selected in such away as to bring the centre of gravity of the cables closer to thegeometrical centre of the conduit 101 (seen in section) at a distancewhich is preferably less than (D-d)/6, where D represents the internaldiameter of the conduit 101 and d represents the external diameter ofone of the cables 102 a, 102 b, 102 c. Thus it is possible to obtain agood compromise between the reduction of the magnetic induction (FIG. 7)and the limitation imposed by the overall dimensions of the system ofcord and cables inside the conduit 101.

[0082] For installing the line 100, the first step is normally to laythe conduit 101, by excavating a trench and then burying individuallengths of conduit in succession. The individual lengths are then joinedby welding or another method which ensures electrical continuity andimpedes the ingress of moisture. The trench can then be reclosed,allowing a rapid restoration of the area affected by the laying. Thecables are then threaded into one end of the conduit and pulled from theother end. In the embodiment illustrated in FIG. 1, the cables 102 a,102 b, 102 c are brought together in a trefoil configuration, and thenthe cord 103 is wound in a spiral around them, to prevent their movementwith respect to each other; they are then threaded into the conduit 101.When they have been threaded into the conduit, the cables 102 a, 102 b,102 c rest on the cord 103, being positioned with their centre ofgravity in the proximity of the geometrical centre of the conduit 101.During the cable laying stage, the cord 103 is subject to considerabletraction as a result of the weight of the cables 102 a, 102 b, 102 c,and the friction with the bottom of the conduit 101: for this reason,the material from which the cord 103 is made must be able to withstandboth the traction and the abrasion due to rubbing. Preferably, the cordis made from dielectric material. In one embodiment constructed by theapplicant, the supporting cord consists of nylon fibres; other materialswhich can be used are polyester fibres or aramid fibres such as Kevlar®.

EXAMPLE 1

[0083]FIGS. 3 and 4 show the results of a simulation carried out by theapplicant by a finite elements calculation: this evaluated the magneticinduction B at ground level generated by a 400 kV 1500 A transmissionline comprising three cables with a cross section of 2500 mm², arrangedin trefoil inside a ferromagnetic steel conduit with a closed section,having an external diameter of 406 mm and a thickness of 10 mm, buriedin a trench with a depth of 1500 mm. The simulation was carried out withdifferent types of non-grain-oriented steel. The electrical losses inthe conduit were also evaluated: these comprise both losses due to thecurrents induced in the conduit by the current flowing in the cables,and losses associated with the magnetic hysteresis of the material fromwhich the conduit is made.

[0084] Table 1 shows the chemical composition (the percentages relate tothe total weight of the steel), the electrical resistivity and the indexG (ASTM E-112) characterizing the grain size of the steels in question.The co-ordinates of the peak of the magnetic curve of the steels arealso shown. TABLE 1 Type of I II III IV V steel Cfr. Inv. Inv. Inv. Inv.Element Carbon % 0.180 0.130 0.160 0.009 0.010 Manganese % 1.460 0.4900.680 0.200 0.190 Silicon % 0.270 0.200 0.030 0.030 0.030 Sulphur %0.018 0.010 0.011 0.005 0.011 Phosphorus % 0.015 0.011 0.007 0.005 0.005Chromium % 0.050 0.050 0.013 0.013 0.013 Nickel % 0.030 0.020 0.0230.021 0.025 Molybdenum % 0.020 0.010 0.005 0.005 0.005 Aluminium % 0.0270.011 0.055 0.023 0.027 Copper % 0.020 0.100 0.025 0.007 0.016 Total %2.090 1.032 1.009 0.318 0.332 impurities Iron % 97.910 98.968 98.99199.682 99.668 Electrical Ω · m 2.16E−07 1.74E−07 1.66E−07 1.19E−071.17E−07 resistivity G (ASTM E-112) 9.5 7.5 9.5 7.0-7.5 6.0-6.5 μ_(max)482 718 737 956 1114 H_(μmax) A/m 1350 616 970 540 523

[0085] To evaluate the losses due to hysteresis, the angle of hysteresiswas also measured and found to be approximately 30° for all the steelsshown in Table 1.

[0086]FIG. 8 shows a photograph, taken through an optical microscope, ofa portion of type V steel, showing its grains. As can be seen, thegrains do not have a preferred direction of alignment.

[0087]FIG. 3 shows the magnetic induction B at ground level, in μT,calculated by the simulation for the five types of steel in Table 1.

[0088] As can be seen, the conduit made from steel I is clearly theworst of the five considered, with values of magnetic induction B fromthree to seven times higher than those found with steels II to V. Thisresult indicates that this steel is not suitable for screening themagnetic fields generated by a high-power transmission line, if valuesof magnetic induction at ground level of 0.2 μT or less are to beobtained.

[0089] Returning to FIG. 3, it can be seen that, on the other hand,steels II to V are very effective in terms of the screening of themagnetic field, provide values of magnetic induction as low asapproximately 0.1-0.2 μT. It should be noted that values of magneticinduction 100 to 200 times lower are obtained than with a configurationconsisting of three buried cables with no magnetic screen.

[0090]FIG. 4 shows the electrical losses (E) in the conduit, measured inW/m, for the five types of steel considered. As can be seen, steels IIto V are also significantly better than the type I steel in respect ofthis parameter, which is fundamental to the design of the transmissionline, since it is responsible for any overheating of the system. Anincrease in the temperature of the tube due to the flow of parasiticcurrents, in fact, leads to a heating of the cables, with a consequentdecrease of the current-carrying capacity of the cables.

[0091]FIG. 5 shows the magnetic curves measured for steelsI-II-III-IV-V, in other words the curves describing the variation of therelative magnetic permeability μ_(r) as a function of the appliedmagnetic field H. The measurement was made according to the IEC 404“Magnetic materials” standard.

[0092] As can be seen, the abscissa H_(μmax) of the peak of the magneticcurve for type II-III-IV-V steels corresponds to lower values of themagnetic field than the abscissa H_(μmax) of the peak of the curve ofthe type I steel. The values (μ_(max), H_(μmax))) for steels I to V areshown in Table 1.

[0093] Additionally, the type II-III-IV-V steels have a peak of magneticpermeability μ_(max) which is higher than the peak of the curve of thetype I steel.

EXAMPLE 2

[0094]FIG. 6 shows the result of a simulation carried out by theapplicant, in which the conduit was made from a non-grain-orientedlow-carbon steel, containing approximately 2% silicon. FIG. 6 shows thevariation of the magnetic induction at ground level as a function of thethickness (t) of the conduit. As it can be seen, a thickness of only 3mm of silicon steel is sufficient to screen the magnetic field enough toobtain values of magnetic induction at ground level of less than 0.2 μT.With a thickness of 8 mm, it is possible to achieve values of magneticinduction of the order of 10⁻³ μT, at the limit of the range of themeasuring instruments.

1. Method for screening the magnetic field generated by an electricalpower transmission line (100) comprising at least one electrical cable(102 a, 102 b, 102 c), the said method comprising the steps of:inserting the said cable in a conduit (101) comprising at least onelayer of a ferromagnetic material, characterized in that saidferromagnetic material is non-grain-oriented and has a magnetic curvewith a maximum value of relative magnetic permeability (μ_(max))corresponding to a magnetic field value (H_(μmax)) lower than 1000 A/m.2. Method according to claim 1, characterized in that the said magneticcurve has a maximum value of relative magnetic permeability (μ_(max))corresponding to a magnetic field value (H_(μmax)) in the range from 10A/m to 800 A/m.
 3. Method according to claim 1, characterized in thatthe said magnetic curve has a maximum value of relative magneticpermeability (μ_(max)) corresponding to a magnetic field value(H_(μmax)) in the range from 30 A/m to 650 A/m.
 4. Method according toany one of the preceding claims, characterized in that the said maximumvalue of relative magnetic permeability (μ_(max) ) is at least
 500. 5.Method according to any one of the preceding claims, additionallycomprising the step of: burying the said conduit (101) in a trench ofpredetermined depth.
 6. Method according to any one of the precedingclaims, characterized in that the said layer is produced by extrusion.7. Method according to claims 1 to 5, characterized in that the saidlayer is produced by the bending of a sheet of predetermined dimensions,and the subsequent welding of the sheet along its longitudinallyopposing sides.
 8. Method according to claim 7, characterized in thatthe said sheet is produced by rolling.
 9. Method according to any one ofthe preceding claims, additionally comprising the step of: arranging thesaid at least one cable in the said conduit (101) in such a way that thecentre of gravity of a cross section of the said at least one cable isin the proximity of the geometrical centre of a corresponding section ofthe conduit (101).
 10. Method according to any one of the precedingclaims, additionally comprising the step of: winding at least oneelongated element (103) around the said at least one cable. 11.Electrical power transmission line (100), comprising: a conduit (101)comprising at least one layer of ferromagnetic material, at least oneelectrical cable (102 a, 102 b, 102 c) inside the said conduit (101),characterized in that the said ferromagnetic material is of thenon-grain-oriented type and has a magnetic curve, having a maximum valueof relative magnetic permeability (μ_(max)) corresponding to a magneticfield value (H_(μmax)) lower than 1000 A/m.
 12. Electrical powertransmission line according to claim 11, characterized in that the saidmagnetic curve has a maximum value of relative magnetic permeability(μ_(max)) corresponding to a magnetic field value (H_(μmax)) in therange from 10 A/m to 800 A/m.
 13. Electrical power transmission lineaccording to claim 12, characterized in that the said magnetic curve hasa maximum value of relative magnetic permeability (μ_(max))corresponding to a magnetic field value (H_(μmax)) in, the range from 30A/m to 650 A/m.
 14. Electrical power transmission line according toclaims 11 to 13, characterized in that the said maximum value ofrelative magnetic permeability (μ_(max)) is at least
 500. 15. Electricalpower transmission line according to claim 11, characterized in that thesaid ferromagnetic material is a steel whose total content of impuritiesdoes not exceed 1.5%.
 16. Electrical power transmission line accordingto claim 15, characterized in that the said steel has a total content ofimpurities not exceeding 1%.
 17. Electrical power transmission lineaccording to claim 16, characterized in that the said steel has a totalcontent of impurities not exceeding 0.5%.
 18. Electrical powertransmission line according to claim 15, characterized in that the saidsteel is a low-carbon steel.
 19. Electrical power transmission lineaccording to claim 18, characterized in that the said steel has a carboncontent not exceeding 0.16%.
 20. Electrical power transmission lineaccording to claim 19, characterized in that the said steel has a carboncontent not exceeding 0.03%.
 21. Electrical power transmission lineaccording to claim 15, characterized in that the said steel has amanganese content not exceeding 1%.
 22. Electrical power transmissionline according to claim 21, characterized in that the said steel has amanganese content not exceeding 0.5%.
 23. Electrical power transmissionline according to claim 11, characterized in that the said ferromagneticmaterial is a silicon steel.
 24. Electrical power transmission lineaccording to claim 23, characterized in that the said steel has asilicon content is in the range from 1% to 4%.
 25. Electrical powertransmission line according to claims 11 to 24, characterized in that itcomprises a support (103) for the said at least one cable (102 a, 102 b,102 c) inside the said conduit (101).
 26. Electrical power transmissionline according to claims 11 to 24, characterized in that it comprises anelongated element wound in a spiral around the said at least one cable(102 a, 102 b, 102 c).
 27. Electrical power transmission line accordingto claim 26, characterized in that the said elongated element is a cordmade from dielectric material.
 28. Electrical power transmission lineaccording to claim 27, characterized in that the said dielectricmaterial is selected from a group comprising nylon fibres, aramid fibresand polyester fibres.
 29. Electrical power transmission line accordingto claims 11 to 28, characterized in that the thickness of the saidconduit (101) is in the range from 2 to 14 mm.
 30. Electrical powertransmission line according to claim 29, characterized in that thethickness of the said conduit (101) is in the range from 4 to 10 mm. 31.Electrical power transmission) line according to claim 30, characterizedin that the thickness of the said conduit (101) is 8 mm.
 32. Electricalpower transmission line according to claims 11 to 31, the said cableshaving an external diameter D_(c), the said conduit having an internaldiameter D_(t), characterized in that D_(t) is in the range from 2.3D_(c) to 2.8 D_(c).
 33. Electrical power transmission line according toclaim 15, characterized in that the resistivity of the said steel isless than 20 μΩ·cm.
 34. Electrical power transmission line according toclaim 15, characterized in that the grain size index G (ASTM E-112) ofthe said steel is less than 9.